Method and apparatus for influencing the output voltages of current supply installations

ABSTRACT

A method and apparatus for influencing or controlling the vectors of output voltages from a direct-current supply device feeding a multiconductor alternating-current transmission system. The total voltage vector for each conductor is generated from two partial voltage vectors. Control of the magnitude of the total voltage vector is effected by phase-shifting the partial voltage vectors, each partial voltage vector being phase-shifted through the same angular magnitude but in opposite angular direction. In this manner, the total voltage vector for each conductor is maintained in a constant phase position even during automatic regulation and in the presence of an asymmetrical load.

United States Patent lnventors Werner Ullman;

Franco Donati, Locarno; Gianfranco Tortelli, Ascona, Switzerland Appl. No. 668,619 filed Sept. 18, 1967 Patented Feb. 9, 1971 Assignee A.G. Fur lndustrielle Elektronik Agie Losone B. Locarno Losone-Locarno, Switzerland a corporation of Switzerland Priority Sept. 27, 1966 Switzerland 13960/66 METHOD AND APPARATUS FOR INFLUENCING THE OUTPUT VOLTAGES OF CURRENT SUPPLY INSTALLATIONS Primary Examiner-William M. Shoop, Jr. AttorneyJacobi, Davidson and Kleeman ABSTRACT: A method and apparatus for influencing or controlling the vectors of output voltages from a direct-current supply device feeding a multiconductor alternating-current transmission system. The total voltage vector for each conductor is generated from two partial voltage vectors. Control of the magnitude of the total voltage vector is effected by phase- 15 Claims 9 Drawing Figs shifting the partial voltage vectors, each partial voltage vector US. Cl 321/5, being phase-shifted through the same angular magnitude but 321/27 in opposite angular direction. In this manner, the total voltage Int. Cl I102m 7/00 vector for each conductor is maintained in a constant phase Field of Search 321/5, 27, position even during automatic regulation and in the presence 27MS of an asymmetrical load.

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Q I l =90'+170) Q ('90 %=90+1a') U0 (1 Voltage mail/afar) 10i! v INVENTORS METHOD AND APPARATUS FOR INFLUENCING THE OUTPUT VOLTAGES OF CURRENT SUPPLY INSTALLATIONS BACKGROUND OF THE INVENTION The present invention relates to an improved method of and apparatus for influencing or controlling the vectors of output voltages from a current supply installation or device during the presence of an asymmetrical load such as caused by at least one load or consumer connected through a two-conductor or multiconductor transmission system with the current supply device. The basic-current supply device referred to above essentially consists of a direct-currentsource, control devices and static inverters which receive ignition pulses from the control devices.

Loads drawing relatively high power preferably receive their current supply through a polyphase system, for example 'a three-conductor transmission system. Such a system has a predetermined voltage and a particular frequency. When loads of different voltage or different frequency requirements are connected to an existing supply system, corresponding converting means must be attached to the load. Such converting means are likewise required when a permanent current supply to polyphase loads is to be provided, forexample by means of a battery, in the event of failure of the normal supply system or in the event of any other disturbances therein such as harmonics, switching surges, etc. These converting means are either rotary converters consisting of a generator and a motor, or static converters consisting of a direct-current source or a rectifier arrangement and inverters. Such converters operate satisfactorily if a symmetrical load is connected within the multiconductor system. However, if an asymmetrical load is present within the multiconductor system, i.e. if a different load is connected or disconnected between each conductor, an asymmetry of the angles present between the vectors occurs in relation to the external loading because of the appreciable internal impedances of the inverters. If differential loads, for example those presented by a radar apparatus or electrical laboratory apparatus or large-scale dataprocessing installations, are connected to the multiconductor system, costly regulating precautions must be taken, because such loads depend upon a satisfactory current supply with constant values of the polyphase relations despite any asymmetrical loading.

Accordingly, it is a primary object of the present invention to provide an improved method and apparatus wherein the voltage vectors for the individual conductors of a multiconductor supply are maintained constant in their phase position in relation to one another despite asymmetrical loading. This constant phase position is to be maintained even in automatic regulation.

Additionally, it is desired to ensure that the current supply of the instant invention can be connected where necessary in parallel operation to existing alternating-voltage supply systems or to other circuit arrangements according to the invention, while the frequencies of the output alternating voltages may be adjustable as desired and according to choice, depending upon the load requirements.

SUMMARY OF THE INVENTION The principle of a current supply system consisting of rectifiers and inverters for feeding supply conductors, the inverters receiving striking or ignition pulses from striking circuits controlled by an oscillator, is adequately set forth and illustrated in British Pat. application No. 47,998 filed Nov. 1 l, 1965, now Pat. No. 1,095,029, and in Austrian Pat. No. 257,746.

Proceeding from this principle, the present inventive method and apparatus fulfills the aforementioned requirements and objects in that the inventive method and apparatus are characterized by the features that, the ignition impulses which are transmitted from one control device to an associated inverter are displaced by the phase angle I /2, and

the ignition impulses transmitted from another control device to another associated inverter are displaced by the phase angle I /2. Both phase-displaced inverter outputs are combined to produce an output voltage for associated conductors. ln this manner, the voltage vectors between the conductors of a multiconductor system may be adjusted to a fixed phase relation to one another. This is particularly important because otherwise the phase relation between conductors would change during regulation as a result of asymmetrical loading.

It is advantageous to maintain this phase relation constant by this technique in many applications, for example, in the current supply of radar equipment constituting an asymmetrical loading, and fed with alternating voltage at 400 c/s. The circuit arrangement and method of the instant invention are also suitable for the current supply in aircraft and for the current supply of ground stations for air traffic. For measurement purposes in laboratories, a current supply at any variable frequency is often desired.

Alternatively, in accordance with the subject invention, the voltage vectors may be arbitrarily changed, or controlled, in their mutual phase relations. This control is desirable when, for example, an elliptical rotating field is to be present in a three-conductor system instead of a circular rotating field, for the purpose of carrying out particular tests on a load such as synchronous or asynchronous motors, for example.

Additionally, a fine-stage speed control of an asynchronous or synchronous motor may be effected by means of the circuit arrangement according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, two embodiments of the invention will be more fully explained with reference to the drawings, in which:

FIG. 1 illustrates a part of the circuit arrangement mentioned in the above patent and patent applications;

FIGS. 1a, 1b and 1c show the vector diagrams of the individual conductor voltages in a three-conductor transmission system.

FIG. 2 illustrates a circuit arrangement according to the invention for carrying out the inventive method;

FIGS. 2a, 2b and 2c depict the vector diagrams of the individual conductor voltages in a three-conductor transmission system; and

FIG. 3 illustrates a further circuit arrangement for carrying out the inventive method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings, it will be recognized that in FIG. 1 an oscillator l is arranged in a control device 9. Oscillator 1 controls a monostable multivibrator 2 which, via a Schmitt-trigger 3, acts upon a flip-flop circuit 4. The latter delivers control pulses to ignition circuits 7,8. Oscillator l is directly coupled to ignition circuits 10, 11. The ignition circuits 7, 8, and 10, 11 are electrically coupled with inverters 5 and 6 respectively. Controlled rectifiers 9,126,122,123 in the inverter 5 are ignited in pairs in accordance with the igni- 1 the actual voltage at the load. The magnitude of the differential signal, and thus, the phase displacement of the voltage E with respect to the voltage U,, is proportional to the deviation of the reference voltage from the actual voltage. The inverter 6 composed of controlled rectifiers 132,142,135J39, is controlled by the ignition impulses from the ignition circuits 10,11. The partial voltage 5, at the output of inverter 6 and produced from the direct-current voltage source [3, is likewise of squarewave-shape but is not phase displaced with i'espect to the control impulses of the Oscillator 1. With additive coupling of both of these partial voltages "E,, E for instance by means of transformers 143,144 at the outputs of the inverters 5,6, there results a stepped voltage from which it is possible to form a sinusoidal voltage by means of, for instance, a subsequently connected filter arrangement. This alternatingcurrent voltage appears across the conductors 150 and 151 of a two-conductor transmission system. This system is explained in greater detail in British Pat. No. 1,095,029 andin the Austrian Pat. 257,746, so that only the essential details have been considered herein. 1

FIG. 1a shows the vector U of the sinusoidal alternating voltage for a two-conductor transmission system. The vector is obtained by vectorial addition of the two component voltage vectors E E the vector E having the phase angle D in relation to the vector E The voltage vector U,,,, is at an angle B in relation to the voltage vector'lJ, of the oscillatorl. In order that the voltage U,,, may be maintained constant in its value with variable voltage conditions at the load 149, the angle 1 of the component voltage E is changed through the differential amplifier 14, the control unit 9 and the inverter 5. At the same time, however, the angle B also changes. With a two-conductor arrangement, however, this is not critical. The disadvantage that the phase B of the voltage vector U also changes on voltage regulation because ofconditions at the load, takes effect only with a three-conductor transmission system and with asymmetrical loading.

For a better understanding; the three voltage vectors U U and U between the neutral conductor and the conductors l, 2, 3 are shown in FIG. 1b. These vectors of the total voltage are at l to one another. The vector U is composed of the component vectors E E, of the two inverters acting on the conductor 1. The vector U is obtained by vectorial addition of the two component voltages E2,E2'. The latter component voltages are generated by the two inverters acting on the conductor 2. The vector U is formed of the vectors E E of the two inverters acting on the conductor 3. Therefore, six inverters are present for the formation of the circular rotating field, the inverters acting in pairs on one conductor. In each pair, only one inverter can carry out the phase shift through the angle 1 The voltage vectors E,,,E, ,E, thus form the phase angles 1 D 4 with their associated voltage vectors E ,,E ,E, With the symmetrical load assumed in FIG. 1b, these angles are I I D Consequently, the angles 3,, B [3 between the vectors U of the individual conductor voltages and the vectors of the oscillator voltages U U U are equal. Each pair of inverters acting on one conductor is therefore constructed as illustrated in FIG. 1. Consequently it comprises one oscillator and one control unit with striking circuits. The oscillators apply their control pulses at intervals of 120 to the multivibrator 2, so that a circular rotating field according to FIG. lb with an angular velocity u) is set up. The voltages between the individual conductors, which are also known as interlinked voltages, are equal in their value and in their angle to one another.

This is apparent from FIG. lb if the peaks of the voltage vectors U U U are joined together. This has not been done in the drawing in order not to impair its'clarity. The described conditions also remain constant on regulation in dependence upon symmetrical voltage changes at the load, because the angles b Q I are changes in the same way. Consequently, the angles B,, B B are always equal to one another.

If a asymmetrical load, however, is applied between the indit idual conductors in the three-conductor system, a vector diagram according to FIG. lcis obtained, in which the vectors of the componentvoltages E E between the conductor 1 and the neutral conductor, E E between the conductor 2 and the neutral conductor, and E il between the conductor S and the neutral conductor have been-added in the same way therein to form the vectors Um, U U of the total voltage existing in each conductor; In accordance with the asymmetrical loading of each conductor, the "angle D is automatically dition, the vectors of the component voltages no longer have the same value. This is a'pparentfrom the different lengths of these vectors, in contradistinction to FIG. lb. FIG. 10 further shows that, due to automatic regulation, the individual conductor voltage vectors U U U areregulated to the same amount, but the interlinked voltages which represent the vectorial combination between the peaks of the individual conductor vectors are no longer equal to one another. Therefore, acircular rotating fieldwhich rotates with the angular velocity (0 is no longer present.

FIG. 2 illustrates a circuit arrangement according to the instant invention in which an oscillator 1 acts on two control units 9,12. Each control unit is of like construction. Therefore, the same reference numerals have been chosen. The oscillator controls a'monostable multivibrator 2, a Schmitttrigger 3, a flip-flop circuit 4, and two striking circuits 7,8,10,11 in the two control units 9,12 The latter apply striking pulses to controllable rectifiers 1l9,'l26,122,l23 in the inverter 5 and to controllable rectifiers 132,142,135J39 in the inverter 6. The inverters 5,6 are connected to a direct-current source 13, Connected to the monostable multivibrator 2 in' the control unit 9,12 is,the differential amplifier 14, which is con nected to the load or loads 149. Depending upon the dif' ference between the desired and actual voltages at the load,

the control pulses are phase-shifted in relation to the pulses of the oscillator l in each monostable multivibrator 2. The differential amplifier 14 so controls the two monostable multivibrators 2 that the upper one applies control pulses shifted by the angle I /2 and the lower one applies control pillses shifted through the angle 90 l l2 to the sequentially connected Schmitt-trigger 3, the flip-flop circuit 4 and the striking circuits'4,10. These striking circuits apply their striking pulses to the rectifiers 119,126 in the inverter 5 and to the rectifiers 132,142 in the inverter 6. The other striking circuits 8,11 apply striking pulses staggered through l8 0 to the rectifiers 122,123 in the inverter 5 and to the rectifiers 135,139 in the inverter 6, in dependence upon the first-mentioned striking circuits. Therefore, both in the inverter 5 and in the inverter 6, rectangular component voltages E,, E: are produced from the direct current source 13, which are phaseshifted in relation to thecontrol pulse in the oscillator 1 by the phase angle 90 1 /2 and 90 4V2. Each inverter has a transformer 143,144 as its output. There is set up at the transformer 143, the component voltage E and at the transformer 144, the component voltage E For a better comparison, the voltage U, of the control pulses of the oscillator 1 has also been shown. Since the two component voltages E,,E are added together because of the seriesconnection of the transformers 143,144, astepped total voltage is obtained between the conductor 151 and the neutral conductor 150. A sinusoidal conductor voltage is produced from the stepped voltage by a nonillustrated sequentially connected filter.

In FIG. 2a, the voltage vector U for the conductor voltage between the conductor 151' and the neutral conductor is shown. The two vectors of the component voltages E,, E of the inverters 5,6 are arranged symmetrically in relation to the voltage vector U,,,,. The" vector of the component voltage E hasa phase shift angle a 90 l /2 arid the vector of the component voltage E 'has a phase shift angle'y 90 1 l2 in relation to the vector of the oscillator pulse U Depending on component voltages alwayschange symmetrically in relation to the vector U FIG. 2b shows the vector diagram in a three-conductor transmission system with symmetrical loading in each conductor. The three-conductor transmission system is produced by triplicating the circuit arrangement of FIG. 2. It is unnecessary for the oscillator l to also be triplicated. Alternatively, a single oscillator l for the control units 9,12 may be provided in triple construction. It is essential for the oscillator or oscillators to emit three control pulses at intervals of 120 with a fixed frequency of, for example, 50 c/s. In the case of a three-conductor system, three differential amplifiers are employed, so that the difference between the actual and desired voltages in each conductor is separately determined and utilized for the regulation. In the vector diagram of FIG. 2b, the same reference numerals are employed for the component voltages E of the total voltages U,,,, and the oscillator pulse voltages U,,

for the three conductors 1, 2 and 3 as in FIG. lb. In accordance with the regulation by the differential amplifiers 14, the angles 04,, 'y, for the conductor 1 and the angles (1 y, for the conductor 2, and the angles a y for the conductor 3 are so adjusted that the vector U of the voltage in the conductor 1, the vector U of the voltage in the conductor 2, and the vector U of the voltage in the conductor 3 are always perpendicular to the associated vector of the oscillator voltage U U U The absolute values of the vectors for the component voltages E and for the total voltages U,,,, are the same for each conductor. The same applies to the interlinked voltages between two conductors. A circular rotating field is thus produced, which rotates at the angular velocity 0).

However, if an asymmetrical load is present in a three-conductor system constructed in accordance with the instant invention, the conditions in each conductor do not change despite the regulation by the differential amplifiers 14. This is apparent from FIG. 20, in which a highly asymmetrical loading is shown. The same circuit arrangement is provided as has been briefly explained with reference to FIG. 2b. It is thus a question of a triple arrangement of the circuit arrangement of FIG. 2. For the vector diagram of FIG. 20, the same reference numerals have been chosen. In the case of the asymmetrical loading, the absolute values of the component voltage vectors E and their angles a and y differ considerably in the individual conductors because of the automatic regulation by the differential amplifiers 14. Nevertheless, the interlinked voltages between the individual vectors of the conductor voltages U,,,, are constant. With the circuit arrangement according to FIG. 2, therefore, the great advantage is obtained that the voltages remain constant both in their angle and in their absolute value regardless of the differing regulation in each conductor.

In the foregoing, reference has been made to the regulation of the asymmetrical load through the differential amplifiers 14. The adjustment of the frequency of the alternating voltage appearing at the output of the inverters has not been mentioned. Of course, the oscillator 1 may apply its control pulses to the control unit with any desired repetition frequency, so that loads such as radar equipment, for example, may be directly connected through a multiconductor system. Since the oscillator 1 can continuously change its repetition frequency, it is also possible to control the speed of one or more asynchronous or synchronous generators.

In FIG. 3, there is shown a practical example of a circuit arrangement according to the invention. There is denoted by a consumer or load which may be connected to the circuit arrangement through a three-conductor transmission system. The load represents, for example, a computer in which a particularly high asymmetrical loading is often present between the three conductors. The computer 15 may be connected both to the conductors R, S, T of the inverter pairs 17,18,19 and to the conductors R, S, T of an existing supply system through a three-pole switch 16. This changeover is of advantage when the load 15 is to be energized. Since the circuitclosing current may reach a value which is a number of times (even more than six times) the value of the rated current, the voltage would drop by more than 30 percent during the circuit-closing time in the case of current supply from the inverter pairs 17,18,19. The computer, however, could comprise a supervisory device which immediately cuts off the current supply at a voltage reduction of about 30 percent provided only that the reduction lasts one cycle (for example at 50 c/s or 20 ms.) or longer. In order to avoid this cutoff, the computer 15 is first connected to the supply system 20. This is effected by closing of the switch 21. The changeover switch 16 lies in the illustrated position at the contacts 160, 16c, 16d.

In order to ensure a synchronous changeover, oscillator 27 is permanently connected to the conductor R of the supply system 20 and applies control pulses to the frequency multiplier 28 at a repetition frequency of, for example, 50 c/s. There is setup at the output of this frequency multiplier the third harmonic oscillation at a frequency of 150 c/s which is applied to the three oscillators I. These three oscillators are constructed in this example as a so-called ring counter. The outputs of the ring counter are connected to the control units 9,12. The construction of the control units is apparent from FIG. 2. There are setup at each of the three outputs, control pulses of 50 c/s with a mutual shift of 120. The inverter pairs 17,18,19 thus 1 receive striking pulses of 50 c/s staggered at 120 and, as already described with reference to FIG. 2, generate from the direct-current supply system 13, which may be a direct-current source 22 and/or a rectifier arrangement 23 supplied by an AC voltage source, the desired conductor voltages and interlinked voltages according to FIG. 2b. As already described with reference to FIG. 2, there are provided at the outputs of the inverter pairs 17,18,19 transformers whose secondary windings are connected, for example, in star. These transformers as well as filters are indicated in FIG. 3 only by the references 17a, 18a, 19a. In FIG. 3, the conductors R, S, T' are connected in star with the neutral conductor N(O). The load 15 is, for example, also connected in star. Provided at each pair of inverters is a differential amplifier 14 with its connection between the conductors and the control units 9,12.

It will now be assumed that the inverter pairs 17,18,19 generate the desired voltage under no load. A phase discriminator 24 situated between a conductor, for example R, of the supply system 20 and the corresponding conductor, for example R of the circuit arrangement monitors whether the voltage vectors ofthe supply system and of the circuit arrangement have the same angle. If there is no agreement between the angles, the phase discriminator 24 applies a difference signal to the phase shifter 25, whereby the oscillator 27 is so regulated that the inverter pairs 17,18,19 generate at the conductors R, S, T conductor voltages and interlinked voltages whose values and angles are the same as those of the conductor voltages and interlinked voltages at the conductors R, S, T of the supply system 20. When agreement is reached, the zero discriminator 26 receives a signal from the phase discriminator 24. The relay 29 receives current and energizes the relay coil of the changeover switch 16 by means of its contact. The changeover switch 16 then changes over from the supply system 20 to the inventive circuit arrangement. This changeover takes place without any interruption, the conductors R, S, T of the supply system 20 being briefly connected together with the conductors R, S T of the circuit arrangement through the contacts l6e, 16a; l6b,16f; 16c, 16d during the changeover action, which connection may be brought about by means of a special contactor or by two independent contactors. As soon as the changeover switch lies on the contacts l6e,l6b,l6c, the computer 15 receives its current supply through the circuit arrangement and is thus independent of the mains supply system. The switch 21 remains closed. The oscillator 27 continues to be synchronized with the supply system 20 through the phase discriminator 24 and the phase shifter 25. In this case, one speaks of a mains-commutated inverter. If the computer 15 must continue to be operated even during a sudden failure of the mains, it is readily possible to sever the oscillator 27 from its mains control and to construct it as a freely oscillating oscillator. In this case, one speaks of a self-commutating inverter, because the oscillator 27 applies its control pulses to the striking circuits of the inverters 17,18,19

independently of the mains. This possibility of a self-commutating inverter and thus the function thereof as a permanent current supply means is not, however, shown in detail in FIG. 3. in any case, the differential amplifiers l4 regulate any asymmetrical load between the conductors R, S, T, as already described in FIG. 2. It is therefore immaterial whether the system functions as a mains-commutated inverter or as a selfcommutating inverter. The phase shifter 25 has a fixed phase shift of 90 in relation to the voltage vectors in the conductor R of the supply system 20. This is necessary because of the special voltage regulation in the circuit arrangement.

It should now be apparent that the objects set forth at the outset of this specification have been successfully achieved.

We claim: 7

l. A method for producing multiphase electric power comprising the steps of: providing a source of direct-current power; inverting the direct-current power to provide two wave components for each phase of the multiphase output; controlling the two wave components such that the wave components are displaced a variable amount by equal phase angles in opposite phase directions, the displacements being considered relative to a reference phase angle; and automatically maintaining the equality of the phase angle displacements.

2. A method for producing multiphase electric power, said method comprisingthe steps of: providing a source of direct current voltage; inverting the direct current voltage under the control of ignition pulses to provide two voltage vector components for each output power phase; displacing the ignition pulses for one of the vector, components in one phase direction to a variable extent and displacing the ignition pulses for the other vector component in the opposite phase direction to an equal extent, both displacements being considered relative to a reference phase angle; and, automatically maintaining the equality of both phase displacements.

3. A method as defined in claim 1, wherein the two wave components for each output phase are combined to generate an output voltage vector of variable magnitude and constant phase.

4. A method as defined in claim 2, wherein the two wave components for each output phase are combined to generate an output voltage vector of variable magnitude and constant phase.

5. A method as defined in claim 1, wherein the output power is produced in the presence of an asymmetrical load and is supplied thereto by means of a multiconductor transmission system, each of the output phases being supplied to a respective conductor of the system.

6. A method as defined in claim 1, wherein the reference phase angle defines a displacement which is the same in direction and extent for both wave components and is constant for each of the output power phases.

7. A method as defined in claim 2, wherein the reference phase angle defines a displacement which is the same in direction and extent for both wave components and is constant for each of the output power phases.

8. An apparatus .for supplying multiphase electric power, said apparatus comprising: direct current input means, output means for the supply power having a plurality of phases, in-

verter means for converting said input direct current into two wave components for each of the phases, and control means for displacing said two wave components to a variable extent by equal phase angles and opposite phase directions and for automatically maintaining the equality of the phase angle displacements, both of the displacements being considered relative to a reference phase angle.

9. An apparatus for supplying multiphase electric power. said apparatus comprising: direct current input means. output terminal means for the supply power having a plurality of phases, inverter means controlled by ignition pulses for converting said input direct current into two voltage vector components for each of the phases, and control means for generating said ignition pulses such that said ignition pulses for one of the voltage vector components are displaced in one phase direction to a variable extent, and such t at said ignition pulses for the other of said voltage vector components are displaced in the opposite phase direction to an equal extent, and for automatically maintaining the equality of said phase displacements, both displacements being considered relative to a reference phase angle.

10. An apparatus as defined in claim 8, further including means for each output phase for combining the two components to generate an output voltage vector of constant phase and of magnitude variable by said control means 11. An apparatus as defined in claim 9, further including means for each output phase for combining said two components to generate an output voltage vectorof constant phase and of magnitude variable by said control means.

12. An apparatus as defined in claim 9,'wherein aid inverter means are controlled by ignition pulses generated by said control means, said control means comprising a multivibrator, oscillator means for supplying control impulses to said multivibrator, and differential amplifier means for providing an error signal indicative of the difference between a desired load voltage magnitude and the actual load voltage magnitude and the actual load voltage magnitude and for applying the error signal to said multivibrator to cause a phase shift of said ignition pulses of each control means relative to the control impulses of said oscillator means.

13. An apparatus as defined in claim 12, wherein two inverter means are provided for each of said output phases, one control means being provided for supplying to one of said inverter means ignition impulses displaced in one phase direction, another of said control means supplying to the other of said inverter means ignition impulses displaced by an equal amount in the opposite phase direction, both displacements being relative to a reference phase angle.

14. An apparatus as defined in claim 13, wherein said reference phase angle defines a phase displacement.

15. An apparatus as defined in claim 13, wherein each of said inverter means includes four control rectifiers arranged in pairs, each of said control means including two ignition circuits, each of said ignition circuits being connected to a respective one of said pairs of control rectifiers, and wherein each of said control means includes a monostable multivibrator coupled to a Schmitt-trigger and a flip-flop to control one of said ignition circuits. 

1. A method for producing multiphase electric power comprising the steps of: providing a source of direct-current power; inverting the direct-current power to provide two wave components for each phase of the multiphase output; controlling the two wave components such that the wave components are displaced a variable amount by equal phase angles in opposite phase directions, the displacements being considered relative to a reference phase angle; and automatically maintaining the equality of the phase angle displacements.
 2. A method for producing multiphase electric power, said method comprising the steps of: providing a source of direct current voltage; inverting the direct current voltage under the control of ignition pulses to provide two voltage vector components for each output power phase; displacing the ignition pulses for one of the vector components in one phase direction to a variable extent and displacing the ignition pulses for the other vector component in the opposite phase direction to an equal extent, both displacements being considered relative to a reference phase angle; and, automatically maintaining the equality of both phase displacements.
 3. A method as defined in claim 1, wherein the two wave components for each output phase are combined to generate an output voltage vector of variable magnitude and constant phase.
 4. A method as defined in claim 2, wherein the two wave components for each output phase are combined to generate an output voltage vector of variable magnitude and constant phase.
 5. A method as defined in claim 1, wherein the output power is produced in the presence of an asymmetrical load and is supplied thereto by means of a multiconductor transmission system, each of the output phases being supplied to a respective conductor of the system.
 6. A method as defined in claim 1, wherein the reference phase angle defines a displacement which is the same in direction and extent for both wave components and is constant for each of the output power phases.
 7. A method as defined in claim 2, wherein the reference phase angle defines a displacement which is the same in direction and extent for both wave components and is constant for each of the output power phases.
 8. An apparatus for supplying multiphase electric power, said apparatus comprising: direct current input means, output means for the supply power having a plurality of phases, inverter means for converting said input direct current into two wave components for each of the phases, and coNtrol means for displacing said two wave components to a variable extent by equal phase angles and opposite phase directions and for automatically maintaining the equality of the phase angle displacements, both of the displacements being considered relative to a reference phase angle.
 9. An apparatus for supplying multiphase electric power, said apparatus comprising: direct current input means, output terminal means for the supply power having a plurality of phases, inverter means controlled by ignition pulses for converting said input direct current into two voltage vector components for each of the phases, and control means for generating said ignition pulses such that said ignition pulses for one of the voltage vector components are displaced in one phase direction to a variable extent, and such that said ignition pulses for the other of said voltage vector components are displaced in the opposite phase direction to an equal extent, and for automatically maintaining the equality of said phase displacements, both displacements being considered relative to a reference phase angle.
 10. An apparatus as defined in claim 8, further including means for each output phase for combining the two components to generate an output voltage vector of constant phase and of magnitude variable by said control means.
 11. An apparatus as defined in claim 9, further including means for each output phase for combining said two components to generate an output voltage vector of constant phase and of magnitude variable by said control means.
 12. An apparatus as defined in claim 9, wherein aid inverter means are controlled by ignition pulses generated by said control means, said control means comprising a multivibrator, oscillator means for supplying control impulses to said multivibrator, and differential amplifier means for providing an error signal indicative of the difference between a desired load voltage magnitude and the actual load voltage magnitude and the actual load voltage magnitude and for applying the error signal to said multivibrator to cause a phase shift of said ignition pulses of each control means relative to the control impulses of said oscillator means.
 13. An apparatus as defined in claim 12, wherein two inverter means are provided for each of said output phases, one control means being provided for supplying to one of said inverter means ignition impulses displaced in one phase direction, another of said control means supplying to the other of said inverter means ignition impulses displaced by an equal amount in the opposite phase direction, both displacements being relative to a reference phase angle.
 14. An apparatus as defined in claim 13, wherein said reference phase angle defines a 90* phase displacement.
 15. An apparatus as defined in claim 13, wherein each of said inverter means includes four control rectifiers arranged in pairs, each of said control means including two ignition circuits, each of said ignition circuits being connected to a respective one of said pairs of control rectifiers, and wherein each of said control means includes a monostable multivibrator coupled to a Schmitt-trigger and a flip-flop to control one of said ignition circuits. 